Abstract:

A solid oxide fuel cell structure obtainable by selective electro-chemical
processing, comprising an electrolyte membrane (4) formed by a thin film
more than 50 nm but less than 10 μm thick, covering a supporting
structure (1) made of a bulk substrate, the supporting structure having
at least one 100 μm to 100 mm wide opening that is covered by the
electrolyte membrane (4). A metallic grid (9) is applied over the
electrolyte membrane (4) and serves at the same time as structural
element to support the membrane and as current collector. The metallic
grid (9) has gridlines that are higher than the membrane thickness and
whose height to width ratio is larger than 0.5, and a mesh size about 10
to 1000 times smaller than the width of said opening. The metallic grid
(9) can be applied on top of a patterned metallic sub-layer structure
(7,7a) arranged for supplying a fine distribution of current and
increasing the density of electrolyte-electrode boundaries exposed to the
fluid. The structure is useful for current generation and as a gas
sensor.

Claims:

1. Solid oxide fuel cell structure obtainable by selective electrochemical
processing, the structure comprising:an electrolyte membrane formed by a
thin film more than 50 nm but less than 10 μm thick, covering a
supporting structure made of a bulk substrate, the supporting structure
having at least one opening that is 100 μm to 100 mm wide and that is
covered by the electrolyte membrane; anda metallic grid applied over the
electrolyte membrane and serving at the same time as structural element
to support the membrane and as current collector, the metallic grid
having gridlines that are higher than the membrane thickness and whose
height to width ratio is larger than 0.5.

2. Structure of claim 1, wherein the metallic grid has a mesh size about
10 to 1000 times smaller than the width of said opening.

3. Structure of claim 1, wherein:the metallic grid is applied on top of a
patterned metallic sub-layer structure,the sub-layer structure comprises
a wider pattern corresponding to the applied metallic grid, and a finer
pattern that is not covered by the applied metallic grid, said finer
pattern being arranged for supplying a fine distribution of current and
increasing the density of electrolyte-electrode boundaries exposed to the
fluid, andthe metallic grid is on an anode side which in use is exposed
to hydrocarbon fluid.

4. Structure of claim 1, wherein the metallic grid is an anode current
collector, and a current collector structure is added on an opposite
cathode side.

5. Structure of claim 4, wherein the cathode current collector structure
has the same size and shape as the anode current collector grid.

6. Structure of claim 4, wherein the cathode current collector structure
has a different shape and/or is displaced with respect to the anode
current collector grid.

7. Structure of claim 4, comprising a first electrical contact to the
metallic anode grid, the first contact being located on an anode side of
the structure, and a second electrical contact to the cathode, the second
contact also being located on the anode side of the structure

9. Structure of claim 1, wherein the metallic grid is made from at least
one metal selected from Ni, Cu, Pt, Cr, Mo, Ag, Co, and Pd.

10. Structure of claim 1, further comprising a porous anode film in
particular selected from NiOx and Ni-(electrolyte) composite.

11. Structure of claim 1, further comprising a porous cathode film in
particular selected from a porous mixed conductor such as
(LaSr)(CoFe)O3 (LSCF) or La1-xSrxCoO3 (LSCO); and
composites of an electronically conductive oxide and an ionically
conductive oxide.

12. Structure of claim 1, wherein the grid exhibits a polygonal pattern
including in particular regular polygons such as triangles, squares,
hexagons and octagons.

14. An array of structures of claim 1, which are fabricated on a common
substrate and connected electrically to form an array of solid oxide fuel
cells.

15. A method of producing the structure claim 1 by photolithographic
patterning and electrochemical, physical and chemical vapour processing
of successive layers on an etchable substrate, wherein the membrane and
the metallic grid are applied to the substrate, and the substrate is
etched to provide said opening.

16. The method of claim 15, wherein the substrate opening is etched by dry
etching in SF6 gas or by wet etching in alkaline solutions.

17. The method of claim 15, wherein the metallic grid is moulded in a
polymer mould, in particular of photoresist.

18. A gas sensor comprising a structure according to claim 1, wherein the
structure is exposed to gas and generates a voltage as a function of gas
pressure.

Description:

[0002]The essential part of a solid oxide fuel cell (SOFC) consists of a
ceramic plate made of an oxygen ionic conductor having the function of a
solid electrolyte. The electrolyte plate is covered on the anode side by
a porous metallic film, and on the cathode side by an electronic
conductive oxide. Being exposed to a flow of air or oxygen (O2),
this cathode layer takes up oxygen, and supplies the necessary electrons
to form oxygen ions that traverse the electrolyte plate to reach the
anode. There, a hydrocarbon gas mixture is oxidized by the oxygen ions
and the electron charges are given to the anode electrode. The respective
electrochemical potentials on the two sides are such that a voltage
difference of roughly one Volt is installed allowing for recuperation of
electrical power. The operation temperature of a classical SOFC amounts
to 800 to 1000° C. (see e.g. B. C. H. Steele, A. Heinzel,
Materials for fuel-cell technologies, NATURE vol. 414, p. 345 (2001)).
Operation procedures, application fields, and design depend crucially on
the operation temperature. While high operation temperatures are good for
large cells working continuously, high temperatures are unpractical for
small cells and for automotive applications. In the first case, heat
losses become too important, and in the second case, the time and energy
consumption for start-up become too large. It is conceivable to operate
SOFC's at temperatures as low as 500° C. (R. Doshi, V. L.
Richards, J. D. Carter, X. Wang, and M. Krumpelt, Development of SOFCs
that operate at 500° C., J. El. Chem. Soc. vol. 146, p. 1273
(1999)). Such low temperatures are compatible with concepts of small SOFC
cells in the centimeter dimension, able to be employed as energy source
for portable electronic and electric devices. In such markets,
miniaturized SOFC's are expected to have a large potential. Micro-SOFC's
would be fuelled by liquid butane for instance.

[0003]In miniaturized cells of lower temperature, the electrolyte plate or
layer thickness must be reduced to reduce resistive losses inside the
electrolyte. The ionic conductivity a follows an exponential law of the
form:

σ = A T exp ( - E a / kT ) ( 1 )

where A is a constant, Ea the activation energy, k the Boltzmann
constant and T the absolute temperature. The internal resistance per unit
area Ri of the cell can be written as:

R i = t el σ = t el T A exp ( E a / kT
) ( 2 )

[0004]Ri is typically chosen as 0.1-0.2 Ωcm2 to allow for
currents of 100 mA/cm2 without loss of output voltage. Taking as an
example a Ce0.8Gd0.2O3 membrane, whose activation energy
amounts to 0.7 eV, the thickness must be reduced by a factor 21 when
decreasing the temperature from 1273K to 823K, provided that the
conductivity of the material is the same for both thicknesses. This means
that the electrolyte membrane cannot be anymore a self supported one of
several 100 μm to 1 mm thickness, but must be thinned down to 5-50
μm and supported by another structure. Such supporting structure
described in the literature is a 300 to 1200 μm thick porous anode,
consisting usually of a composite structure of nickel and yttrium
stabilized zirconia (YSZ) (see, e.g. P. Holtapples, U. Vogt, and T.
Graule, Ceramic materials for advanced solid oxide fuel cells., Adv. Eng.
Mat. vol 292, p. 292 (2005)).

[0006]In US patent application US 2005/0115889-A1 (pub. date Jun. 2, 2005)
entitled "Stressed thin film membrane islands", a stiffening structure is
proposed to increase the stability of thin film membranes that are
closing a large opening in a substrate. The proposed solutions include a
grid structure superimposed to the thin film membrane to stiffen and
support the membrane. The solution proposed is based on silicon
micromachining techniques including deep silicon etching to define
trenches. These are filled with nitride and oxide materials by means of
thermal or plasma enhanced chemical vapor deposition. There are three
major problems related to that invention: [0007]1) The membrane needs
to be perforated to grow the supporting structure, risking leakage in
case of SOFC application. [0008]2) The supporting structure traverses the
membrane, requiring insulating material for the supporting structure.
[0009]3) The supporting structure is produced in a rather complicated,
delicate and costly way.

[0010]The invention aims to remove some of these problems, and add
additional functionality.

SUMMARY OF THE INVENTION

[0011]In this invention, a different way is pursued. The membrane thinness
is chosen to allow for compatibility with thin film and micromachining
technology. An electrolyte membrane in the thickness range of 50 nm to 10
μm--deposited for example by sputtering or sol-gel techniques--closes
a 100 μm to 100 mm wide opening of a substrate of typically 0.5-1 mm
thickness. A metallic grid structure with a mesh size that is usually 10
to 1000 times smaller than the substrate opening supports the membrane to
avoid cracking caused by excessive stresses and buckling. In addition,
the metallic grid serves as current collector, preferentially on the
anode side.

[0013]The invention thus provides a solid oxide fuel cell structure
obtainable by selective electrochemical processing, the structure
comprising: [0014]an electrolyte membrane formed by a thin film more
than 50 nm but less than 10 μm thick, covering a supporting structure
made of a bulk substrate, the supporting structure having at least one
opening that is 100 μm to 100 mm wide and that is covered by the
electrolyte membrane; and [0015]a metallic grid applied over the
electrolyte membrane and serving at the same time as structural element
to support the membrane and as current collector, the metallic grid
having gridlines that are higher than the membrane thickness and whose
height to width ratio is larger than 0.5, preferably larger than 1 and
even more preferably larger than 2.

[0016]The metallic grid usually has a mesh size about 10 to 1000 times
smaller than the width of said opening.

[0017]By virtue of the grid's morphology and its aspect ratio, more space
is left for the porous anode material which is easier to access. The new
metallic grid is relatively thick and open, allowing better opportunities
for 3-phase contact leading to better performance. It is also adapted to
the expected thermal strains and stresses.

[0018]Preferably, the metallic grid is applied on top of a patterned
metallic sub-layer structure, the sub-layer structure comprising a wider
pattern corresponding to the applied metallic grid, and a finer pattern
that is not covered by the applied metallic grid, this finer pattern
being arranged for supplying a fine distribution of current and
increasing the density of electrolyte-electrode boundaries exposed to the
fluid, and the metallic grid is on an anode side which in use is exposed
to hydrocarbon fluid.

[0019]Usually, the metallic grid is an anode current collector, and a
cathode current collector structure is added on an opposite cathode side.
The cathode current collector structure can have the same size and shape
as the anode current collector grid, or the cathode current collector
structure can have a different shape and/or be displaced with respect to
the anode current collector grid.

[0020]The invention removes some of the aforesaid problems with US
2005/0115889-A1, and adds additional functionality to the supporting
structure: [0021]1) The membrane (electrolyte in our case) is not
perforated during fabrication of the supporting structure. [0022]2) The
supporting structure is on one side only (anode) and can be made with a
metallic material and with high aspect ratio. [0023]3) The fabrication is
less complicated and less expensive, as it uses electrochemical
deposition of typically nickel. The mould for the creation of high aspect
ratio grid structures is the not the silicon substrate on which the
membrane is grown (as in US 2005/0115889), instead we can use a patterned
thick photoresist on top of the membrane to form the mold. [0024]4) The
metal grid is used at the same time to work as current collector, thus
supporting the functionality of thin porous electrodes, as typically used
on the anode side. [0025]5) A further extension introduces a thin
metallic network structure on the side opposed to the supporting metallic
grid to collect the current on this side and connect to a contact pad on
the same side as the other one. The side with the contacts is
preferentially the anode side, thus in reducing atmosphere, in order to
avoid oxidation of contact metals.

[0026]A critical issue is the stability of membranes at the operation
temperature (500-600° C.). Typically-used electrolyte materials
such as CeGdO2 (CGO) or YSZ exhibit a large thermal expansion of
around 12 ppm/K, which in addition may depend on the oxygen partial
pressure. The thermal expansion of Ni comes quite close (13 ppm/K). Hence
the Ni grid and the electrolyte membrane expand by about the same amount
when the temperature is raised. The substrate usually has much less
expansion (Silicon: 3 ppm/K, silicon glasses: 1 to 8 ppm/K). Having a
larger thermal expansion than the substrate, the membrane will buckle.
The critical buckling strain

crit = π 2 t 2 3 L 2

(S. P. Timoshenko, and J. M. Gere: Theory of elastic stability,
McGraw-Hill, N.Y. 1961, pp 49) for a 1 μm membrane closing a 10 mm
diameter opening amounts to less than 1 ppm. A temperature change of
500° C.--as occurs when installing between room temperature and
operation temperature--results in a thermal strain
(εop=(αmem-αsub)ΔT) of up to
0.5%, thus by far larger than the critical strain. The role of the
metallic grid is to partition the membrane into smaller areas exhibiting
larger critical strains (for instance, a 1 μm membrane within a 100
μm wide opening of the grid exhibits a critical strain of already
0.3%). The Ni grid takes up the forces from the border of the large
opening in the substrate. The grid being thicker than the membrane
(usually more than twice as thick) the buckling--if occurring--is
smoother and more regular than that of the thin electrolyte membrane (see
FIG. 5-1 as an example) and the amplitude is lower. In addition, a metal
grid is much tougher than a ceramic membrane. For large openings, a
warping cannot be avoided, even with a grid of high aspect ratio. In this
case a judicious design of grid will allow for a controlled warping.

[0027]The grid geometry can be adapted to expected strains. Typically the
supporting grid exhibits 5 μm wide and 10 μm high grid lines (i.e.
with the aspect ratio, height to width, of 2) and defining 20 to 200
μm wide grid openings (mesh size). A high aspect ratio ensures keeping
a large efficiency of the cell, which is proportional to the active area
exposed to the fuel gas flow divided by total area (can be called
"filling factor"). Membrane thickness and diameter of grid openings can
be matched to obtain locally flat membranes within the grid openings, and
relax thermal stresses to form a global deformation of the grid/membrane
structure. Engineering to obtain predefined buckling may yield the
requirement that the optimal grid geometry at the border of the membrane
might be different from that in the center of the membrane. This is
anyhow true for designs inspired by spin webs. In case of a hexagonal
grid, the border elements could be filled with triangles to reinforce
stiffness at the border (as in FIG. 5-1).

[0028]The metallic grid plays at the same time the role of current
collector. Being at the anode side, a porous Ni-electrolyte composite is
deposited on top of the membrane on the grid side. The metallic grid
guarantees the global connectivity of the porous layer, and allows
reduction of the electrical conductivity of the porous electrode
material. Preferential material for the grid is material that is well
grown by electrochemical deposition, and in addition compatible with the
anode-side function, such as nickel, palladium, copper, molybdenum,
cobalt, ruthenium, iridium. Suitable seed layer materials are: Pt, Ir,
Ru, etc., possibly also nickel or copper.

[0029]Furthermore, the sub-layer or seed layer used for the
electro-deposition of the metallic grid can be extended to provide a fine
distribution of current. Only parts of this seed layer are then covered
by the resist forming the mould for electro-deposition. This fine
distribution may be a regular sub-grid structure. Its mesh size may be
decreased to reach a high density of fine distribution lines, thus
approaching an artificial porous structure. The fine distribution lines
could be as thin as a few 100 nm's. They could be organized as fractal
structure carrying the current form the inside of the grid opening to the
grid lines, having in the center a higher density of very narrow lines,
and towards the grid line, a lower density of wider lines.

[0030]The metallic grid may exhibit a polygonal pattern including in
particular regular polygons such as triangles, squares, hexagons and
octagons or irregular shapes including spider web type shapes and fractal
structures.

[0031]The invention also concerns an array of structures as described,
which are fabricated on a common substrate and connected electrically to
form an array of solid oxide fuel cells.

[0032]The SOFC structures of the invention are useful for
current-generating applications as well as applications where they are
used to generate a potential difference, e.g. when the structure is used
as a gas sensor, exposed to gas in small concentrations at, say,
400-500° C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]The invention will be further described by way of example with
reference to the accompanying drawings, in which:

[0034]FIG. 1-1 shows an embodiment of a fuel cell structure according to
the invention, without a cathode collector grid;

[0035]FIG. 1-2 shows another embodiment of a fuel cell structure according
to the invention with a cathode collector grid;

[0036]FIG. 2 illustrates in FIGS. 2-1 to 2-16 the successive steps for the
fabrication of the structure of FIG. 1-2;

[0037]FIG. 3 illustrates in FIGS. 3-1 to 3-4 variations where the anode
grid is grown on a sub-layer or seed layer;

[0038]FIG. 4 is a schematic top view showing the pattern of the metallic
anode grid, sub-layer or seed layer and current collector;

[0039]FIG. 5 shows in FIGS. 5-1 and 5-2 photographs of examples of
different grid shapes; and

[0040]FIG. 6 shows a cross sectional view of a planar oxygen sensor.

DETAILED DESCRIPTION OF THE DRAWINGS

[0041]For convenience, the reference numbers used in the drawings are
shown in Table 1.

[0042]FIG. 1-1 is a schematic view of a PEN structure of a fuel cell
composed of a porous cathode 12, a thin electrolyte film 4 and porous
anode layers 10. The PEN is mounted on a substrate 1 and mechanically
supported by a metallic grid 9a which is also a part of the anode current
collector. Electrical connections 2b and 9b respectively for the anode
and the cathode are both placed on the anode side of the device.

[0043]FIG. 1-2 also shows a PEN structure of a fuel cell like that of FIG.
1-1 but additionally with a current collector grid 2 on the cathode side.

[0044]FIG. 2 illustrates the process flow steps for the fabrication of the
device shown in FIG. 1-2. Steps 1 to 3 are skipped for the device shown
in FIG. 1-1. The process steps of FIGS. 2-1 to 2-16 are as follows:

[0045]FIG. 2-1: Deposition of a metallic layer 2 of thickness t1 by PVD on
a substrate 1 of thickness to. Layer 2 will constitute the current
collector on the cathode side and the electrical contact for the cathode.

[0046]FIG. 2-2: Deposition and structuration of a photo-sensible polymer
layer 3 serving as first photoresist mask for the current collector 2
etch.

[0047]FIG. 2-3: Dry etching of the current collector 2 and removal of the
photoresist mask 3. Layer 2 is now a mesh with line width w1 and line
spacing w2. On the border of the membrane, an electrical contact 2b of
dimension L1 is also structured.

[0049]FIG. 2-5: Deposition and structuration of a photo-sensible polymer
layer 5 serving as second photoresist mask for the electrolyte 4 etch.

[0050]FIG. 2-6: Dry etching of an opening ("via hole") of dimension L2 in
the electrolyte 4 and removal of the photoresist mask 5. The "via hole"
serves for making electrical contact with the cathode 2 via its contact
2b.

[0051]FIG. 2-7: Deposition and structuration of a photo-sensible polymer
layer 6 serving as third photoresist mask for the lift-off of the seed
layer 7.

[0052]FIG. 2-8: Deposition of a metallic layer 7 of thickness t3 by PVD
over the photoresist mask 6. Layer 7 serves as seed layer for the
electrodeposition and as current collector for the anode, depending on
the design shown in FIG. 3.

[0053]FIG. 2-9: Removal of the photoresist 6. Seed layer 7 is structured
by lift-off with line width w3 and line spacing w4. A gap of dimension L3
is not covered by layer 7. This gap serves as via for the electrical
connection of the cathode 2. On one border, the membrane 4 is covered by
layer 7 over a dimension L4. The seed layer 7 is connected to an
electrical contact on the border of the substrate for electrodeposition.

[0054]FIG. 2-10: Deposition and structuration of a photo-sensible polymer
layer 8 serving as mould for the electrodeposited layer 9 (9a and 9b).

[0055]FIG. 2-11: Electrodeposition of the metallic grid 9a and contact 9b
of thickness t4. The grid 9a has a line width w5 and a line spacing w6
(FIG. 2-12). The metallic grid 9 also serves as electrical contact 9b for
the anode covering a length L4 of the seed layer 7.

[0056]FIG. 2-12: Removal of the photoresist mould 8. The thickness of the
metallic grid is t4, the line width w5 and the space between lines w6.

[0057]FIG. 2-13: Deposition of the porous anode layer 10 of thickness t5
with a hard mask protecting the electrical contact of the cathode.

[0059]FIG. 2-15: Dry etching of an opening of size w5 in the substrate 1.

[0060]FIG. 2-16: Deposition of a porous cathode layer 12 of thickness t6
on the backside of the substrate 1.

[0061]FIG. 3-1 is a schematic cross-sectional view of a part of the
metallic grid 9 supporting the electrolyte 4, anode 10 and cathode 12. In
this example, the whole sub- or seed-layer 7 is covered by the metallic
grid 9. The current collector of the cathode 2 is placed under and only
under each grid element.

[0062]FIG. 3-2 is like FIG. 3-1 where in addition some parts 7a of the
sub- or seed-layer 7 are not covered by the metallic grid 9 and serve as
current collector for the anode 10.

[0063]FIG. 3-3 is like FIG. 3-1, where a current collector of the cathode
2 is introduced that has its lines at the same position as the lines of
the grid 9a and additional grid lines 2a in-between.

[0064]FIG. 3-4 represents a combination of FIGS. 3-2 and 3-3. Parts 7a of
the seed layer 7 are not covered by the metallic grid 9 and serve as
current collector for the anode 10, and some parts 2a of the current
collector of the cathode 2 are not at the same position as the metallic
grid 9. In this case, the extra lines 7a of the seed layer 7 and 2a of
the cathode 2 can overlay one another (as shown) or can intersect with
one another.

[0065]FIG. 4 is a schematic top view of the metallic grid 9, seed layer 7
and current collector of the cathode 2, showing the extra seed-layer
lines 7a and the extra cathode collector lines 2a. The line widths of the
three networks are between 0 μm and 50 μm and not necessarily
equal. The spaces between the lines of these three networks are between 1
μm and 500 μm and not necessarily equal. The centres of the cells
of the different networks can be super-imposed or displaced by distances
between 1 μm and 100 μm.

[0066]FIG. 5-1 is a photograph of an example of a nickel grid of
triangular shape on top of a free-standing CGO membrane. Side length of
triangles: 50 μm. In this case, the triangular shapes are arranged to
form a series of hexagons.

[0067]FIG. 5-2 is a photograph of an example of a nickel grid of hexagonal
shape on top of a free-standing CGO membrane. Side length of hexagons: 50
μm. In this photo, the controlled buckling of the membrane in the grid
is clearly visible.

[0068]The invention is a Positive electrode-Electrolyte-Negative electrode
(PEN) structure of a solid oxide fuel cell including an anode grid 9
supporting an electrolyte membrane 4. The invention is used for
mid-temperature to medium temperature range solid oxide fuel cells
(300° C.-600° C.). The originality of the invention is the
grid 9 serving as mechanical support of the thin electrolyte layer 4, as
part of anode and as link for the electrical connections. This design
allows placing the two electrical contacts (anode and cathode) on the
same side of the support and facilitates the current collection. The
supporting grid prevents thermal cracks in the electrolyte membrane 4 and
allows improving the reactive area of the cell. The anode and cathode
triple phase boundary lines (TPL) can also be improved by the inclusion
of micro structured current collector meshes on the both sides of the
electrolyte. The original and easy micro fabrication process of this
structure is also part of the present invention.

[0069]The invention is related to a PEN structure for fuel cell
applications comprising: [0070]a substrate 1 with a "large" opening of
width w5 from 100 μm to 100 mm; [0071]an electrolyte membrane 4
closing the said opening; [0072]a metallic grid 9 supporting the membrane
4 and being part of the anode; [0073]an anode 10; [0074]a cathode 12;
[0075]a current collector mesh formed by the grid 9 on the anode side;
[0076]a current collector mesh 2 on the cathode side; [0077]an electrical
contact 9b to the anode, located on the anode side; and [0078]an
electrical contact 2b to the cathode, also located on the anode side.

[0080]In a first design, the electrolyte 4 is directly deposited onto the
substrate 1 (possibly using a buffer layer) (FIG. 1-1). In a second
design, a metal mesh for cathode current collector 2 is deposited and
patterned first (FIG. 1.1, FIG. 2-1, 2-2, 2-3). It is deposited by PVD,
CVD, evaporation or PLD (pulsed laser deposition) and structured using
photolithography and dry or wet etch. The collector 2 has the form of a
two-dimensional mesh. The collector material is a conductive metal or
oxide. The thickness t1 of the collector 2 is between 50 nm and 200 nm,
its line width between 1 mm and 10 μm and the spaces between lines
between 5 μm and 500 μm.

[0082]The electrolyte membrane 4 is supported by grid 9 (FIGS. 2-10 to
2-12). For its fabrication, a photoresist 6 is deposited and patterned
first (FIG. 2-7) to allow for deposition of a seed layer 7, which is
patterned by dissolving the resist (lift-off technique) arriving at the
schematic structure of FIG. 2-9. The seed layer 7 is an electrical
conductive metal such Cr, Au, Al, Cu, Pt, Pd, Ni, Mo, Ag, Ce, Gd or
combination thereof. It is deposited preferentially by means of
evaporation or magnetron sputtering. In the next step the mould 8 is
prepared in the form of a patterned resist. The grid structure 9 is grown
inside the mould by electrochemical deposition, the current being
supplied through the seed layer 7 (FIG. 2-11), which is connected on the
wafer level to a power supply. Alternatively, the deposition can be
performed by electroless plating. The growth of layer 9 yields the grid
structure 9a, the anode contact 9b, which is of the same body as the
grid. The material of the grid 9 is selected among electronically
conductive metals (including Ni, Cu, Fe, Pt). The height t4 of the grid 9
is usually at minimum twice as thick as the electrolyte membrane 4 and
between 1 μm and 100 μm, its line width w5 between 1 μm and 100
μm. The openings in the grid have dimensions w6 between 5 μm and
500 μm.

[0083]The grid 9 mechanically supports the electrolyte membrane 4 and
serves as current collector for the anode layer 10. The grid 9 covers the
central part of the substrate to reach contacts for external electrical
connections via contact 9b. The anode layer 10 is deposited by PVD, spray
pyrolysis, CVD, PLD or evaporation over and in the spaces of the grid 9,
covering the free surface of the electrolyte 4 (FIG. 2-13). The cathode
contact on width L3 is protected by a sacrificial layer to avoid a short
(by dry or wet etching or by lift-off). The thickness t5 of the anode
layer 10 is between 50 nm and 5 m. The material of the anode 10 is a
porous composite of an electronic conductor (Ni, Pt, Ce, Gd . . . ) and
an ionic conductor (YSZ, CGO, . . . ), or a porous mixed conductor (LSCF,
LSC . . . ).

[0084]The liberation of the membrane 4 is preferentially carried out by
deep dry etching process as available for silicon and silicon glass.
First a thick resist 11 is deposited, and patterned by photolithography
(FIG. 2-14). The resist serves as mask for deep dry etching. An opening
is formed into the substrate 1, of width w5 (FIG. 2-15), measuring 100
μm to 100 mm.

[0085]The cathode layer 12 is deposited by PVD, spray pyrolysis, CVD, PLD
or evaporation over the electrolyte on the opposite side of the anode
through the large opening in the substrate (dimension w5) of the support
(FIG. 2-16). Cathode layer 12 covers the cathode current collector 2, to
which there is automatically an electrical contact. The thickness t6 of
the cathode is between 50 nm and 5 mm. The material of the cathode is a
porous composite of an electronic conductive oxide (IrO2, RuO2)
and an ionic conductor (YSZ, CGO, LSC, LSCF . . . ) or a porous mixed
conductor such as (LaSr)(CoFe)O3 (LSCF) or
La1-xSrxCoO3 (LSCO)

[0086]Variations of the design shown in FIGS. 3-1 to 3-4 relate to the use
of a seed layer 7 as current collector mesh on the anode side, and of a
thin film structure as current collector on the cathode side. In FIG.
3-1, the grid structure 9 is grown on the entire seed layer 7. On both
sides, no fine distribution of current is foreseen. In FIG. 3-2, a part
of the seed layer 7 is used to distribute on a smaller scale the current
within each opening of the grid 9. This fine distribution can be
effectuated by a multitude of electrically connected lines 7a. The
simplest case, with one additional sub-lattice is shown (as in FIG. 4).
In FIG. 3-3, an additional thin current collector 2 is introduced on the
cathode side, one cathode mesh having the same pattern as and being below
the structure 9 of the anode side, and the additional cathode grid 2a
being spaced in between the anode grid 9. As a further variation, the two
current collectors might be shifted with respect to each other (FIG. 4).

[0087]Use as Gas Sensor

[0088]In addition to the generation of electricity, the structure
according to the invention can also be used as a gas sensor wherein the
structure is exposed to gas and used to generate a voltage as a function
of a gas pressure. Theoretically, the output voltage ΔV of a solid
electrolyte stack is given by the expression:

where pO2(1) and pO2(2) are the partial oxygen pressures on the
two sides, R and F are the gas constant and Faraday constant,
respectively. Z is the total charge per gas molecule, i.e. 4 for oxygen
(2xO2-). Sensors using this principle are in use for tuning the
fuel-to-air ratio of engines (Lambda sensors). They are made of bulk YSZ
(Yttria stabilized zirconia). A recent published version with planar
geometry, still using bulk or thick film YSZ, is shown in E. I. Tiffee,
et al, Electrochim. Acta 47 (2001) 807. A lower thermal capacity and a
built-in heater serves to shorten time between start of the engine and
reaching operation temperature of the sensor, as compared to the first
generation k-sensors.

[0089]It is clear that a thin film version of this device according to
this invention would exhibit even smaller thermal capacities, and need
less power to heat the sensor. It could also be used at lower
temperatures.

[0090]Such a thin film gas sensor according to the invention is shown in
FIG. 6. This gas sensor has one electrolyte layer 4 sandwiched between
the anode grid 9a and cathode 2. The substrate 1 is micromachined to
provide a cavity 13 adjacent the cathode side to contain the reference
air or other gas. The pO2 is measured by the voltage between the anode
grid 9a and the cathode 2. A heater 4 is formed as part of the substrate
and can consist of metal wires heated by the Joule effect (see Local
growth of sol-gel films by means of microhotplates, Calame F. Barborowski
J., Ledermann N., Muralt P., Gentil S. and Setter N., Integrated
Microelectrics 54: 549-556 2003.

[0091]One can either measure the open circuit voltage, or the limiting
current in the amperometric mode, at which oxygen ions are pumped through
the ion conductor until a limit is reached depending on the supply of
oxygen or reducing gas (controlled by diffusion barrier) or ions
(controlled by catalytic processes at electrodes). Descriptions of
operation modes and electrolyte behavior in YSZ are for instance found in
R. Ramamoorthy, P. K. Dutta, S. A. Akbar, J. Mat. Science 38 (2003) 4271;
and B. Y. Liaw, W. Weppner, J. Electrochem. Soc. 138 (1991) 2478.

[0092]For an estimation of the sensitivity we consider potentiometric
measurements. In a sensor with a dilute reducing gas, we would have a
small pressure difference. We write the pressures as:

pO2(2)=pref; pO2(1)=pref+Δp.

[0093]As we can linearise the logarithm around one as ln(1+x)=x, we should
measure at 800 K a voltage of:

Δ V = 17.2 [ mV ] Δ p p ref
( 4 )

[0094]This value has to be appreciated in relation to the intrinsic noise
of the cell, and to the characteristics of the electronics. The signal to
noise ratio of the sensor is the crucial quantity to be evaluated. The
noise source can be assumed to be the internal resistance of the element
given by the ionic conduction resistivity. So we would deal with a
Johnson noise, or resistor noise described by:

Vn= {square root over (4kTRΔf)} (5)

[0095]Today CGO membranes can be produced with a conductivity of 100 S/cm
at 500° C. An element with 100 μm square and 1 μm thickness
thus has a resistance R of 0.01*10-4/10-8=100 Ohm. At 800
K, and a bandwidth of 10 Hz one obtains a noise voltage of 7 nV.

[0097]Hence, the sensor has the potential to reach a ppm resolution. No
literature on noise evaluation of solid oxide electrolyte sensors was
found. However, for H2S sensors working by means of a polymer proton
conducting membrane noise limits of less than 100 ppb are reported in G.
Schiavon et al, Anal. Chem. 67 (1995) 318. There is one article on noise
evaluation of oxygen conductors [C. M. Van Vilet, J. J. Brophy, Phys.
Rev. B 47 (1993)11149].

[0098]The simple picture of equation 5 is probably not quite correct. At
low frequencies, the noise is not white (independent of frequency).
Instead, it was found that the noise increases with lowering frequency
like f-3/2. For this reason, the measurement should be
preferentially done in a modulation mode at a frequency above 10 Hz in
order to reduce the noise. The most natural way is to modulate the
temperature. A low thermal capacity and a fully integrated heater are
prerequisites to achieve low thermal time constants, and efficient
heating. Both arguments speak in favour of thin film MEMS structures
according to this invention.

[0099]Equation 3 is strongly modified by the role of the electrodes. The
electrolyte material is not directly exposed to the gases. The effective
oxygen partial pressure is a function of catalytic reactions at the
electrodes, and of the ion exchange at the electrode interfaces, or more
explicitly a the triple line boundaries between gas, electrolyte and
electrode. In the article L. P. Martin, R. S. Glass, J. Electrochem. Soc.
152 (2005), a bulk YSZ hydrogen sensor was investigated yielding 300 mV
at 1% hydrogen. Both sides were exposed to the same gas. The voltage
difference in this case was due to the different effect of Pt electrodes
on the one side, and ITO (indium tin oxide) electrodes on the other one.
At the Pt side oxygen reduction is predominant, while at the ITO side,
hydrogen oxidation. Such phenomena may also allow for a new type of
design, where the electrodes are both on the same side, and the membrane
structure would only serve to reduce the heat capacity.

[0100]For the gas sensor application, the same materials apply as for the
above-described SOFC structure. The nickel grid can however be made
thinner due to the greatly reduced current. Making the structure with
small heat capacity is advantageous to modulate temperature for better
signal to noise ratios compared to the bulk version. Advantageously, for
the gas sensor application the structure will be combined with an on-chip
heating system in view of the fact that at the low gas pressures used
there is insufficient gas consumption to generate the necessary heat at
the operating temperature.